1. Field of the Invention
The present invention relates to thermoplastic packaging films which are capable of forming a peelable heat-seal so that the sealed film may be reopened without tearing or rupturing of the film layers.
2. Description of the Prior Art
In the past, thin thermoplastic films have been widely used for the packaging of a wide variety of commodities and have found particular widespread acceptance in the packaging of foodstuffs. Included in such thermoplastic packaging films are films such as biaxially oriented polypropylene, which is particularly suited for use in such applications because of its excellent optical and mechanical properties. In addition, it offers excellent moisture barrier characteristics. In the past, expanded usage of this type of film for packaging has been hampered somewhat by its inability to offer good heat-seal characteristics over a wide temperature range. One prior art technique which has been employed to remedy this has been the application of heat-sealable coatings onto the film surface after the film has been formed and oriented. However, such coating applications following film orientation are extremely costly and require expensive equipment including drying ovens, solvent recovery systems, and the like.
An alternate means for application of heat-sealable coatings onto the surface of films such as polypropylene, comprises the extrusion of a surface layer of thermoplastic onto a polypropylene film substrate. A thermoplastic coating resin is selected which has a melting point below that of the polypropylene film, so that the laminar assembly may be sealed at lower temperatures. This extrusion step may occur at a stage where the base polypropylene material has already been oriented in one direction, such as the machine direction. In that case, following extrusion coating, the composite assembly is subsequently oriented in the transverse direction. Such films exhibit good heat-seal strengths, but because of the relatively high melting point of the prior art copolymer skin resins, including resins such as ethylene-propylene copolymers, the temperature range over which usable heat-seals may be formed is quite narrow. Moreover, when attempts are made to peel apart layers of such coated films after they have been heat-sealed together, it has been found that during rupture of the heat-seals to gain access to the package contents, the film layers themselves are usually ripped and torn making reclosure of the open package with the original wrapping material difficult or impossible.
It has been known in the past to prepare oriented polypropylene films having a coextruded coating layer comprising low density polyethylene, the polyethylene having been prepared by a free radical catalyzed polymerization process. In this case, because of the lower melting point of the polyethylene skins, a broader sealing range is achieved. However, such films have shown poor adhesion between the polyethylene skin layer and the polypropylene core layer resulting in a tendency for the layers to separate during heat-sealing operations. Consequently, the seal strengths provided by such low density polyethylene skin layers are not as high as the minimum heat-seal strengths required for commercial applications. Moreover, the haze and gloss values of such films are much poorer than those exhibited by the uncoated oriented polypropylene base film.